The use of ultrasonic imaging for medical diagnostic purposes is well known. In particular, ultrasound has been used for many years to aid in the diagnosis of certain cardiac and vascular diseases. In addition, Doppler ultrasound technology has been recognized as an important tool in the evaluation of blood flow information. In Doppler ultrasound imaging, a reflection from a stationary object provides a signal at zero frequency shift (that is, at the intermediate frequency). The Doppler frequency shift in the echo signal returned from a moving target varies monotonically with the instantaneous velocity of the target. A review of cardiac Doppler measurement technology is contained in R. G. O'Connell, Jr., "The Role of Doppler Ultrasound in Cardiac Diagnosis," Hewlett-Packard Journal, June 1986 at pp 20-25, and in P. A. Magnin, "Doppler Effect: History and Theory, Id. at pp 26-31; in L. Halberg et al., "Extraction of Blood Flow Information Using Doppler-Shifted Ultrasound, Id. at pp 35-40; and in B. F. Hunt et al., "Digital Processing Chain for A Doppler Ultrasound Subsystem, Id. at pp 45-48.
A typical prior art medical ultrasound imaging system employs a phased array or mechanical transducer, a scanner unit and a signal processing and display unit. The scanner unit, for example, provides analog signal conditioning, beamforming and signal translation from the ultrasound frequency range to a more convenient intermediate frequency (I.F.) range. The processing and display unit then converts the I.F. signals to digital samples and processes the digital samples in order to facilitate extraction and display of desired information contained in the echo signals.
The display and processing unit may provide both black and white (monochrome) and color imaging information. The monochrome mode typically is used to show anatomic detail, with blood flow shown in the color mode. In a typical system, a two-dimensional monochrome image may show a sector-shaped or rectangular scan region of a patient, displayed at a rate of approximately 30 frames per second. A color mode image may be overlaid on a portion (up to 100%) of the scanned sector, supplementing the monochrome image. At each picture element (pixel) on the display, either the monochrome signal or the color signal is displayed; or alternatively, the two signals may be combined.
The color image is typically a color-coded blood flow map, where the color coding indicates localized velocity and/or turbulence of blood flow. In an exemplary commercial system, velocity is shown in shades of red and blue, with red indicating flow toward the transducer and blue indicating flow away from the transducer, or vice versa. Sometimes another color may be mixed in over a portion of the scale to focus attention on flows within selected ranges. The intensity and/or shading of the color represents the speed of the flow towards or away from the transducer. Shades of green are sometimes added to indicate turbulence.
While the ultrasound image provides a qualitative representation of the region of interest, it is frequently desirable to obtain quantitative measurements of vessel parameters, such as blood velocity, vessel diameter and vessel wall directions. In order to determine blood velocity, the angle between the ultrasound beam direction and the direction of the blood vessel must be determined. A method for adjustment of Doppler angle in ultrasound images is disclosed in European Patent Application No. 0 755 920 published May 28, 1997. This published application describes a technique for calculating the direction of the vessels blood flow and coordinates of the vessel walls in the vicinity of a cursor when the cursor is positioned inside of a vessel in the ultrasound image.
U.S. Pat. No. 5,645,066 to Gandini et al. describes a medical ultrasound imaging system which employs displayed indicators and measure bars to enable the user to better control a displayed image. More particularly, a scanning guide is displayed and changed in accord with which frame of a group of images is being displayed. Further, the scanning guide can show the capacity of an image memory and how much of that capacity has been utilized.
Known techniques for quantitatively determining parameters, such as blood velocity and vessel diameter from ultrasound images have been relatively difficult to use and, even after measurements are taken, it has been difficult to determine their accuracy. The accuracy of such a measurement is dependent upon such factors as the positioning of the scanned ultrasound beam with respect to the structure being imaged. For instance, an accurate measure of the diameter of a vessel must be obtained to assure that the cross-section of the vessel and the blood flow volume therethrough is properly calculated. Once such a calculation is performed, it is thereafter difficult to determine if the transducer was appropriately positioned to enable the measurement of the maximum diameter dimension. Further, such techniques have required relatively skilled operators and do not produce consistent results.
In co-pending U.S. patent application Ser. No. 09/187,013, entitled "Automated Measurement and Analysis of Patient Anatomy Based on Image Recognition" to J. S. Nikom, and assigned to the same Assignee as is this Application, a method is described for measurement and analysis of patient's anatomy which generates an ultrasound image of a region of a patient and provides coordinates of walls of a vessel in the image. One or more parameters of the vessel in the vicinity of a cursor are determined from the wall coordinates and are recorded. The vessel parameters may include vessel diameter, vessel center coordinates and/or vessel wall directions. A center of gravity of an upper vessel wall and a center of gravity of a lower vessel wall are determined from the wall coordinates. The vessel diameter and the vessel center coordinates are determined from the centers of gravities of the upper and lower vessel walls.
The cursor is then to be moved to the vessel center coordinates in the ultrasound image and rotated into alignment with the vessel wall directions. The cursor is then moved along the vessel in the cursor direction to a new position and the process of determining one or more vessel parameters is performed at the new cursor position. By repeating this process, the vessel is automatically mapped. The disclosure of the aforementioned copending patent application is incorporated herein by reference.
To further improve upon the teachings of the above-noted copending patent application, it is desirable to provide methods and apparatus for confirming a user's proper positioning of an ultrasound transducer in relation to a structure being imaged, after the image has been acquired. More particularly, it is desirable to reduce the amount of training necessary to obtain such measurements from ultrasound images; to reduce or eliminate the variability of results from different ultrasound operators; and to enable such measurements to be obtained rapidly.